This application is a national phase application of International Application No. PCT/JP99/01 100, which was filed on Mar. 8, 1999 and which published in Japanese on Sep. 16, 1999, which in turn claims priority from Japanese Application JP 10/56867, which was filed on Mar. 9, 1998, from Japanese Application JP 10/345422, which was filed on Dec. 14, 1998, from Japanese Application JP 11/17410, which was filed on Jan. 26, 1999, and from Japanese Application JP 11/17415, which was filed on Jan. 26, 1998.
The invention relates to a catalyst for removing hydrocarbons from exhaust gas containing methane and an excess of oxygen, and a method for purification of exhaust gas.
In the specification, the xe2x80x9cexhaust gas containing an excess of oxygenxe2x80x9d means that the exhaust gas to be treated with the catalyst of the present invention is an exhaust gas containing oxidizing components such as oxygen and nitrogen oxides in larger amounts than necessary to completely oxidize reducing components which coexist therewith.
Catalysts comprising platinum group metals (e.g., platinum and palladium) supported thereon are known to have high capability to remove hydrocarbons from exhaust gas by oxidation. For example, Japanese Unexamined Patent Publication No. 106691/1976 describes a catalyst for purifying exhaust gas, the catalyst comprising palladium and platinum supported on an alumina carrier. However, even these catalysts fail to produce sufficient purifying effects on exhaust gas containing hydrocarbons which mainly consist of methane having high chemical stability, like exhaust gas resulting from natural gas combustion. Unlike ammonia, methane that is substantially non-toxic and has poor photochemical reactivity is considered not to greatly damage the regional air environment. However, it is desirable that methane emissions be suppressed as much as possible from the viewpoint of future global environmental protection.
It is also known that reaction inhibitors such as sulfur oxides usually coexist in exhaust gas and these compounds greatly reduce catalyst activity with time. Unlike petroleum fuel such as kerosene or light oil, natural gas does not originally contain substantial amounts of sulfur compounds. However, city gas derived from natural gas and supplied in our country further comprises a compound containing sulfur as an odorant, which forms a sulfur oxide during the combustion of gas, inhibiting catalytic activity of platinum group catalysts.
For example, Lampert reports that when methane is oxidized with a palladium catalyst in the presence of only 0.1 ppm of sulfur dioxide in the methane, the catalyst activity is almost totally lost in several hours (Applied Catalysis B: Environmental, vol.14, pp.211-223 (1997)).
Yamamoto et al. report that when hydrocarbons are removed from combusted city gas using a catalyst comprising palladium and platinum both supported on alumina, catalyst activity sharply declines in a short time of about 100 hours (Catalysis Society of Japan Meeting Abstract, 1996, published on Sep. 13, 1996).
Further, Japanese Unexamined Patent Publication No. 332392/1996 describes an oxidation catalyst for removing low-concentration hydrocarbons from exhaust gas in the presence of an excess of oxygen, the catalyst comprising at least 7 g/l of palladium and 3 to 20 g/l of platinum both supported via an alumina carrier on a honeycomb substrate. However, since this catalyst does not have sufficient long-term durability, time-dependent degradation of catalytic activity is unavoidable.
As described above, the conventional exhaust gas treatment methods have the problems that methane removal efficiency is low and that catalyst activity sharply declines in a short time in the presence of sulfur oxides.
Exhaust gas usually contains nitrogen oxides in addition to hydrocarbons. Known methods for removing nitrogen oxides and hydrocarbons from exhaust gas include, for example, a method for removing nitrogen oxides using hydrocarbon as a reducing agent (e.g., Japanese Unexamined Patent Publication No. 310742/1989).
Further, Japanese Unexamined Patent Publication No. 90826/1992 describes a method for purifying exhaust gas, comprising bringing exhaust gas containing nitrogen oxides into contact with a specific catalyst in the presence of hydrocarbon in an oxidation atmosphere containing an excess of oxygen so as to decompose nitrogen oxides and bringing the exhaust gas into contact with an oxidation catalyst so as to suppress hydrocarbon emissions.
According to the methods described in the above publications, propane, propene or the like converts to carbon dioxide at a high rate under conditions suitable for the removal of nitrogen oxides, so that only a low percentage thereof remains in the treated gas. However, a large amount of methane unavoidably resides in the treated gas because of its low reactivity under the conditions where known catalysts for removal of nitrogen oxides exhibit high activity. Therefore, in the case of treating exhaust gas containing methane, improvement is necessary in this point.
Japanese Unexamined Patent Publication No. 90826/1992 describes removing unreacted hydrocarbons by passing exhaust gas through a nitrogen oxide-removing catalyst and thereafter through an oxidation catalyst, and mentions, as oxidation catalysts, catalysts comprising 0.01 to 2% of platinum, rhodium, palladium or the like supported on porous carriers. The publication, however, nowhere describes reactivity of these catalysts to methane. In view of the above-mentioned report of Lampert et al., it is hardly conceivable that such a catalyst is capable of removing methane from exhaust gas.
U.S. Pat. No. 5,260,043 describes a method for removing nitrogen oxides and methane from a gas containing methane, nitrogen oxides and oxygen, the method comprising using a Co-ZSM-5 catalyst in the first stage and using a Pd-ZSM-5 catalyst in the second stage. However, practicability of this method is extremely questionable because neither steam nor sulfur oxides are contained in the gas tested according to the method described in the publication. Furthermore, in consideration of descriptions about effects of steam on methane-oxidation activity of Pd-ZSM-5 catalysts provided in another publication (Yuejin Li and John N. Armor, Applied Catalysis B: Environmental, vol.3, pp.275 (1994)), it is unlikely that methane will be removed from actual exhaust gas containing steam, by the method disclosed in the patent.
The above is summarized as follows. It is difficult to remove methane from actual exhaust gas by oxidation using any prior art technique, by treating the gas within a temperature range where nitrogen oxide-removing catalysts act effectively. It is difficult to achieve a high hydrocarbon conversion rate in the case of treating exhaust gas containing hydrocarbons containing a large amount of methane, for example, exhaust gas resulting from natural gas combustion. Consequently, if natural gas is added to exhaust gas to reduce nitrogen oxides, methane emissions will increase.
Other sources of methane-containing exhaust gas include organism-derived, methane-containing exhaust gas generated by fermentation of garbage, domestic animal wastes and like biological wastes. Such exhaust gas, however, usually contains sulfur-containing organic compounds derived from protein, etc. Therefore, during the removal of hydrocarbons by oxidation with a catalyst, the sulfur-containing organic compounds are oxidized to sulfur oxides, which reduce catalyst activity in a similar manner as described above.
A primary object of the present invention is to provide a catalyst for removing hydrocarbons from exhaust gas containing methane and an excess of oxygen, the catalyst being capable of producing highly purifying effects even on exhaust gas containing hydrocarbons containing a large proportion of methane and exhibiting stable long-term catalytic activity even in the presence of sulfur oxides, and to provide a method for purification of exhaust gas using the catalyst.
The present inventors carried out intensive research to achieve the above object and found the following. A catalyst comprising palladium supported on at least one carrier selected from zirconia, sulfated zirconia (SO4/Zro2) and tungsten-zirconia (WO3/ZrO2) is highly resistant to inhibition of catalyst activity by sulfur oxides and maintains its stably high capability to oxidize methane even in an exhaust gas environment.
The present inventors further found that a catalyst comprising palladium and platinum both supported on at least one carrier selected from zirconia, sulfated zirconia (SO4/ZrO2) and tungsten-zirconia (WO3/ZrO2) is highly capable of oxidizing methane even at lower treatment temperatures.
The present invention has been accomplished based on the above findings. The present invention concerns the following catalysts for purification of hydrocarbon-containing exhaust gas and methods for purification of hydrocarbon-containing exhaust gas.
1. A catalyst for removing hydrocarbons from exhaust gas containing methane and an excess of oxygen, which comprises palladium supported on at least one carrier selected from zirconia, sulfated zirconia and tungsten-zirconia.
2. The catalyst according to item 1 wherein the weight proportion of supported palladium is 2% to 20% relative to the carrier.
3. A catalyst for removing hydrocarbons from exhaust gas containing methane and an excess of oxygen, which comprises palladium and platinum both supported on at least one carrier selected from zirconia, sulfated zirconia and tungsten-zirconia.
4. The catalyst according to item 3 wherein the weight proportion of supported palladium is 2% to 20% relative to the carrier and the weight proportion of supported platinum is 10% to 50% relative to the palladium.
5. A method for removing hydrocarbons from exhaust gas containing methane and an excess of oxygen, using a catalyst comprising palladium supported on at least one carrier selected from zirconia, sulfated zirconia and tungsten-zirconia.
6. The method according to item 5 wherein the weight proportion of supported palladium is 2% to 20% relative to the carrier.
7. A method for removing hydrocarbons from exhaust gas containing methane and an excess of oxygen, using a catalyst comprising palladium and platinum both supported on at least one carrier selected from zirconia, sulfated zirconia and tungsten-zirconia.
8. The method for removing hydrocarbons from exhaust gas according to item 7 wherein the weight proportion of supported palladium is 2% to 20% relative to the carrier and the weight proportion of supported platinum is 10% to 50% relative to the palladium.
9. A method for purifying exhaust gas containing nitrogen oxides and hydrocarbons including methane, the method comprising bringing the exhaust gas into contact with a nitrogen oxide reduction catalyst in the presence of hydrocarbons and thereafter bringing the exhaust gas into contact with a catalyst comprising palladium supported on at least one carrier selected from zirconia, sulfated zirconia and tungsten-zirconia.
10. The method for purifying exhaust gas according to item 9 wherein the weight proportion of supported palladium is 2% to 20% relative to the carrier.
11. A method for purifying exhaust gas containing nitrogen oxides and hydrocarbons including methane, the method comprising bringing the exhaust gas into contact with a nitrogen oxide reduction catalyst in the presence of hydrocarbons and thereafter bringing the exhaust gas into contact with a catalyst comprising palladium and platinum both supported on at least one carrier selected from zirconia, sulfated zirconia and tungsten-zirconia.
12. The method for purifying exhaust gas according to item 11 wherein the weight proportion of supported palladium is 2% to 20% relative to the carrier and the weight proportion of supported platinum is 10% to 50% relative to the palladium.
13. The method for purifying exhaust gas according to any one of items 9-12, which comprises adding natural gas to exhaust gas before bringing the exhaust gas into contact with a nitrogen oxide reduction catalyst.
The catalyst according to the present invention is prepared by having a commercially available zirconia carrier impregnated with a solution containing palladium ion or palladium ion and platinum ion, followed by drying and calcining. The specific surface area of the zirconia carrier is an important factor for achieving stably high dispersion of palladium or palladium/platinum. It is preferable for the specific surface area to be at least 5 m2/g, more preferably in the range of 5 to 50 m2/g. Useful carriers include sulfated zirconia (SO4/ZrO2) and tungsten-zirconia (WO3/ZrO2). Zirconia, sulfated zirconia and tungsten-zirconia may be used singly or in combination of two or more.
Sulfated zirconia and tungsten-zirconia can be prepared by known methods (for example, see xe2x80x9cHyomen(Surface)xe2x80x9d, vol.28, No.7, page 481 (1990); xe2x80x9cHyomen(Surface)xe2x80x9d, vol.34, No.2, page 51 (1996)). Stated more specifically, sulfated zirconia can be prepared by treating zirconium hydroxide with diluted sulfuric acid or causing zirconium hydroxide to be impregnated with ammonium sulfate, followed by drying and calcining. Tungsten-zirconia can be prepared by kneading zirconium hydroxide and tungstic acid in a wet state, followed by drying and calcining.
Hereinafter, xe2x80x9czirconia carriersxe2x80x9d represents all the above three carriers, unless it is especially necessary to mention full names of the particular carriers.
Useful catalytic metal ion-containing solutions include solutions of metal nitrates or ammine complexes of such metals. It is preferable that such solutions be in the form of aqueous solutions but may be solutions of mixed solvents prepared by adding a water-soluble organic solvent such as acetone or ethanol.
The weight proportion of supported palladium is about 1% to about 25%, preferably about 2% to about 20%, relative to the zirconia carrier. If the proportion of supported palladium is too small, low catalytic activity will result. If the proportion of supported palladium is too large, palladium will have a larger particle diameter and will not be used effectively.
In the case of using both palladium and platinum, the weight proportion of supported palladium is the same as described above and the weight proportion of supported platinum is usually about 5% to about 50%, preferably about 10% to about 50%, relative to the palladium. If the proportion of supported platinum is too small, sufficient improvement in effects by combined use will not be achieved. If the proportion of supported platinum is too large, the function of palladium may be suppressed.
Then, the zirconia carrier impregnated with a predetermined amount of the catalytically active component is dried, followed by calcining in air, giving a desired catalyst. To provide stable long-term high catalytic activity, calcination is carried out preferably in the temperature range of about 450xc2x0 C. to about 700xc2x0 C., more preferably about 500xc2x0 C. to about 650xc2x0 C. If the calcination temperature is too high, the specific surface area of the carrier will diminish with the growth of particles of the supported metal, resulting in lower catalytic activity. If the calcination temperature is too low, particles of palladium or palladium and platinum grow during the use of the catalyst, resulting in lower catalytic activity and impairing stability.
The catalyst of the present invention may be used in any form, without limitation. For example, the catalyst may contain a binder and is formed into pellets, or the catalyst may be washcoated on a fire-resistant honeycomb substrate. It is preferable that the catalyst according to the present invention be used as a washcoat on a fire-resistant honeycomb substrate. In the case of washcoating on a fire-resistant honeycomb substrate, the catalyst prepared by the above method may be slurryed and washcoated, or zirconia particles as a carrier may be washcoated on a fire-resistant honeycomb substrate and then have palladium or palladium/platinum supported on the carrier in the same manner as above.
The method for removing hydrocarbons from methane-containing exhaust gas according to the present invention utilizes the catalyst thus obtained. If too small an amount of the catalyst is used for removing hydrocarbons, the desired removal efficiency will not be attained. It is therefore preferable that the catalyst be used in an amount such that the exhaust gas hourly space velocity (GHSV) is 500,000 hrxe2x88x921 or less, more preferably 300,000 hrxe2x88x921 or less. The lower the gas hourly space velocity (GHSV) is, the higher the proportion of the catalyst is, thus increasing removal efficiency and being advantageous. However, using the catalyst at an exhaust gas hourly space velocity (GHSV) of 1,000 hrxe2x88x921 or less is economically disadvantageous and also increases pressure loss in the catalyst bed. Therefore, the amount of the catalyst may be appropriately selected from the range that GHSV becomes 500,000 hrxe2x88x921 or less.
The hydrocarbon-removing catalyst used in the methane-containing exhaust gas according to the present invention is highly active. However, if the purification treatment temperature is too low, the catalyst does not exhibit sufficient catalytic activity, resulting in difficulty in achieving the desired hydrocarbon conversion rate. If the concentration of hydrocarbons in exhaust gas is extremely high, a rapid reaction occurs in the catalyst bed, incurring the danger of adversely affecting durability of the catalyst. In view of these matters, it is preferable that exhaust gas be subjected to purification treatment under reaction conditions such that the temperature of the catalytic layer is maintained in the range of about 350xc2x0 C. to about 600xc2x0 C. and temperature rise (i.e., difference between the catalytic layer outlet temperature and the catalytic layer inlet temperature) is not more than 150xc2x0 C.
In another embodiment of the present invention, a catalyst for reducing nitrogen oxides in the presence of hydrocarbons (hereinafter sometimes simply referred to as xe2x80x9cnitrogen oxide-removing catalystxe2x80x9d) is disposed upstream of a hydrocarbon-removing catalyst, so that both of nitrogen oxides and hydrocarbons can be removed. Useful nitrogen oxide-removing catalysts are not limited and any known one may be used only if it is capable of removing nitrogen oxides by reduction using a saturated lower hydrocarbon as a reducing agent. Examples of useful catalysts include transition metal ion-exchanged zeolite catalysts (e.g., a catalyst comprising cobalt supported on a beta zeolite carrier) and a catalyst comprising silver supported on an alumina carrier.
The amount of the nitrogen oxide-removing catalyst may be selected according to the type of catalyst so as to achieve an appropriate gas space velocity. For example, a catalyst comprising cobalt supported on a beta zeolite carrier is preferably used at a gas hourly space velocity (GHSV) of not 60,000 hrxe2x88x921 or less, more preferably 30,000 hrxe2x88x921 or less. As regards use temperature, for example, a catalyst comprising cobalt supported on a beta zeolite carrier may be used at a temperature ranging from about 300xc2x0 C. to about 600xc2x0 C., preferably about 350xc2x0 C. to about 500xc2x0 C. In usual use conditions, it is impossible to set different use temperatures for a nitrogen oxide-removing catalyst and for a hydrocarbon removing catalyst. Therefore, both catalysts are preferably used in the temperature range that is within a use temperature range suitable for the hydrocarbon-removing catalyst and also within a use temperature range suitable for the nitrogen oxide-removing catalyst.
Nitrogen oxide removal efficiency can be further enhanced by adding natural gas to exhaust gas before bringing the exhaust gas into contact with the nitrogen oxide-removing catalyst in the presence of hydrocarbons. In this case, also, the method of the present invention achieves enhanced nitrogen oxide removal efficiency, while suppressing methane emissions. In other words, the method of the present invention reduces the level of nitrogen oxides and also suppresses methane emissions by providing a nitrogen oxide-removing catalyst upstream of a hydrocarbon-removing catalyst.
Exhaust gas usually contains about 5% to about 15% of steam which inhibits oxidation reaction of methane. The method according to the present invention has high methane conversion effects even on such steam-containing exhaust gas.
Further, exhaust gas usually contains sulfur oxides that are known to greatly reduce catalyst activity. The catalyst according to the present invention is highly resistant to loss of catalytic activity caused by sulfur poisoning, so that stable long-term high hydrocarbon removal efficiency can be achieved.
The catalyst according to the present invention exhibits high methane oxidation activity even in the treatment of exhaust gas containing a large amount of steam, for example, combustion gas, and is highly resistant to catalyst activity inhibition by sulfur oxides. Therefore, the catalyst of the present invention produces stable long-term highly purifying effects even on exhaust gas containing hydrocarbons containing a large amount of methane, whose purification treatment with conventional catalysts has been difficult.
Furthermore, removal of nitrogen oxides and reduction of the level of hydrocarbons are achieved at the same time by disposing a catalyst for removing nitrogen oxides in the presence of hydrocarbons (a nitrogen oxide-removing catalyst) on the upstream side and disposing a catalyst for removing hydrocarbons according to the present invention on the downstream side and passing exhaust gas over the catalysts. With this construction, the method of the present invention is highly effective for purifying exhaust gas containing hydrocarbons containing a large amount of methane, for example, exhaust gas from natural gas combustion. Therefore, methane emissions can be suppressed even when natural gas is added as a nitrogen oxide-reducing agent to the exhaust gas in order to remove nitrogen oxides.